In the oil and gas industry tools are often lowered in a subterranean wellbore by a cable (commonly referred to as a wireline or a wireline cable) for the purpose of monitoring or determining characteristics of the wellbore. Once data is collected by the tool, it is sent from within the wellbore to the surface of the wellbore through the cable. Recently, it has been discovered that optical fibers are able to transmit data from within a wellbore to the surface of a wellbore at a much faster rate than electrical data transmission lines. As such, it is desirable to include optical fibers in oil and gas wellbore wireline cables for the purpose of data transmission. However, several characteristics of optical fibers make them vulnerable to damage in oilfield operations.
For example, exposure to hydrogen at high temperatures results in a “darkening” of optical fibers, which leads to a reduction in data carrying capacity. The difference in linear stretching of optical fibers as compared to the other components of the cable requires additional fiber length to be built in to the optical fiber components, which complicates the manufacturing process. Volatilization of volatile organic compounds (VOCs) in coatings or other polymeric protective layers on the optical fibers releases additional hydrogen which can attack and darken the fibers. Optical fibers are susceptible to hydrolytic attack in the presence of water. A lack of transverse toughness of optical fiber component construction leads to potential point loading and micro-bending issues, which can lead to mechanical failure of the optical fibers and/or increased data attenuation.
One technique used to protect optical fibers from many of the problems listed above is to encase them in a solid metallic tube. However, encasing an optical fiber in a metallic tube has several disadvantages. For example, encasing an optical fiber in a metallic tube is very expensive. End to end welding of metallic tubes, which is necessary to create a wireline cable of a sufficient length, creates difficult-to-detect pinholes. Such welding also produces welding gases, which if trapped inside the tube can lead to deterioration of the optical fibers inside the tube.
In addition, when subjected to torque (which is present in most wireline cables) solid metallic tubes are prone to collapse unless they are excessively thick, as such the tube must be sufficiently thick to prevent collapse under such torque and/or other loads or pressures. However, such added thickness takes up valuable space within the cable core. Also, solid metallic tubes have limited flexibility, and a low fatigue life in dynamic applications; and optical fibers encased in metallic tubes cannot be spliced without over-sizing them. Accordingly, a need exists for an improved method and/or apparatus for encasing an optical fiber in a cable.